Apparatus including electro-optical material for use in testing a circuit having voltage-bearing elements proximate a surface of a body

ABSTRACT

Electro-optical in-circuit testing, especially of large circuits such as those assembled on printed circuit boards, is achieved in an automatic test system by disposing an electro-optical sensor in proximity to the circuit conductors, applying test signals to the circuit under test, and measuring an optical property of the sensor at selected regions thereof corresponding to internal nodes, i.e. test points, of the circuit. The sensor may be an optical probe comprising a lens and a layer of electro-optical material which is adapted to be applied to the circuit. The electro-optical material may be either a polymer film or a crystal, the latter requiring a flexible coupling medium on the face applied to the circuit under test. The electro-optical material is provided with a reflective coating on one surface to facilitate a polarimetric measurement made transverse to the plane of the material. Another type of optical probe comprises a layer of electro-optical material having electrodes on the surface opposite the surface applied to the circuit under test. A polarimetric measurement is made by suitably biasing the electrodes and detecting light that passes through the material parallel to its surfaces. In another type of optical probe, the electro-optical material is coated on both surfaces, one coating being highly reflective and the other being semi-transparent. An interferometric measurement is provided by this type of sensor. Moreover, the polymer film may be applied directly to the printed circuit board during manufacture, in which case the lens is brought into contact with the electro-optical material during circuit testing.

This application is a continuation of U.S. patent application Ser. No.07/630,421, filed Dec. 18, 1990, now U.S. Pat. No. 5,272,434; which is acontinuation of U.S. patent application Ser. No. 07/226,127, filed Jul.29, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to circuit testing, and more particularlyto the testing of circuits using electro-optical means.

2. Description of Related Art

Circuit testing is performed automatically by apparatus which makescontact with the circuit nodes, applies excitation signals to thesenodes, and monitors the response signals produced by the tested circuit.Such apparatus is often called in the art "Automatic Test Equipment"(abbreviated as ATE).

ATE may be classified into two types, namely functional testers andin-circuit testers. In a functional tester, inputs are applied andoutputs received only at the normal input and output nodes of thecircuit, such as for example an edge connector for a board mountedcircuit. Input signals are applied and output signals are monitored toassess the functionality of the circuit assembly as a whole.

A disadvantage of functional testers, particularly where complexcircuits are involved, is that a large number of input/outputcombinations (referred to in the art as test patterns) are required tofully exercise the circuit, and an exhaustive test may be very lengthy.A further problem is that it may be impossible to test some circuitcomponents, notably digital memories and counters, since certain statesof such devices cannot be controlled from externally accessible nodesalone. For example, there may be no external connection to the resetinput of a counter. Equally, some output conditions of some circuits maynot be externally distinguishable. For these reasons, some circuitassemblies can be only partially tested with a functional tester.

On the other hand, in-circuit testers have the advantage of being ableto test each component of a board mounted circuit individually, sincethey are designed to contact internal circuit nodes, that is nodes otherthan the external input and output nodes. However this advantage isobtained at the expense of a special piece of hardware, referred to inthe art as a "bed of nails" fixture, comprising a plurality of springloaded probes, positioned individually to contact nodes within acircuit. By this means component inputs and outputs may be driven andmeasured to establish the functionality of each component individually.

One disadvantage of in-circuit testers is the hardware required inproviding a fixture adapted to each circuit to be tested, for exampleeach board, and means for holding the board in place and in contact withthe fixture. A less apparent disadvantage is possible damage to circuitdevices when an output of a device is driven via a nail to a state otherthan that which would logically result from the signals at its inputs inorder to test another device having an input connected to that output.Such backdriving, as such a circumstance is referred to in the art, candamage the backdriven device by, for example, causing excessive heatingtherein. Further, the miniaturization of circuits increases thedifficulty of accessing the internal nodes of the circuit under test.Moreover, in-circuit testers are intrinsically poorly adapted tomonitor. the overall functioning of a circuit, notably those whichinterfere with the respective timing of signals produced by the variouscomponents.

It is nevertheless desirable that automatic circuit testing be able toprovide contemporaneously analytic information allowing validation ofthe functionality of the components taken individually, and furtherderived information allowing validation of the global functionality ofthe circuit, which depends not only on the functionality of eachcomponent, but also on the interactions between components. Generally,in-circuit testers give priority to the first aspect to the detriment ofthe second, while functional testers do essentially the opposite.

Functional testing may include a "diagnostic phase," a delicateoperation for isolating the probable cause of a fault. It is generallyimpossible to locate some faults by exercising external inputs andmonitoring external outputs alone, since there will not be a unique testpattern corresponding to every possible fault. This problem is to someextent overcome in some functional testers by providing a manual probefor use during the diagnostic phase on boards which have failed thefunctional test. During the diagnostic phase, an operator places theprobe at various nodes of the circuit in such a way that an internalnode may be monitored in a way somewhat similar to that in which itwould be monitored by an in-circuit tester. Such a procedure is slow andnecessitates a skilled operator, however.

The use of electro-optical materials for non-contact monitoring ofelectrical signals has been proposed for integrated circuits in U.S.Pat. No. 4,618,819, which describes a system which uses electro-opticalcrystals placed physically proximate the surface of a not yetencapsulated integrated circuit. A polarized light beam is directedtowards a region of the crystal in the neighborhood of a conductorcarrying an electrical signal to be measured, and then reflected. Inaccordance with well known laws, a characteristic of the reflected lightis affected by the electric field around the conductor, such that byappropriate detection of the reflected light, the electric field and thesignal which produces it may be detected. Thus an image of theelectrical signal in the conductor may be obtained.

Unfortunately, electro-optical systems have not been proposed that aresuitable for use in testing circuits other than integrated circuits,such as, for example, hybrid circuits and circuits assembled on printedcircuit boards.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus whichprovides for electro-optical in-circuit testing of circuits, especiallysuch large circuits as hybrids and circuits assembled on printed circuitboards.

A method for achieving this and other objects comprises the steps ofconformably disposing an electro-optical sensor on the surface of thecircuit under test, so that electro-optical material within the sensoris exposed to electric fields associated with conductors of the circuit.Test signals are applied to the circuit, whereby an optical property ofthe electro-optical material is variously affected by signals on theconductors. The optical property is measured at selected regions withinthe electro-optical material, and a test parameter is determined fromthe result of the measuring step.

An apparatus for achieving this and other objects comprises sensor meanshaving a conformable surface and including electro-optical material;means for disposing the conformable surface of the sensor on the surfaceof the circuit, such that the electro-optical material within the sensoris exposed to electric fields associated with the conductors of thecircuit; means for applying test signals to the circuit, whereby anoptical property of the electro-optical material is variously affectedby signals on the conductors; means for measuring the optical propertyat selected positions within the electro-optical material; and meansresponsive to the measuring means for determining a test parameter.

An embodiment of an interface apparatus for achieving this and otherobjects comprises a layer of electro-optical material having areflective coating upon one of two opposing surfaces, the other surfacebeing transparent to polarized light; and a flexible coupling mediumaffixed to the coated surface of the electro-optical material along oneof the opposing surfaces thereof, the unaffixed opposing surface of saidcoupling medium being adapted to be urged against the surface of thecircuit.

Another such embodiment comprises a layer of an electro-optical polymerhaving a reflective coating upon one of two opposing surfaces, saidcoated surface being adapted to be applied against said circuit surface,and the other surface being transparent to polarized light.

Another such embodiment comprises a layer of electro-optical materialhaving a plurality of discrete electrodes upon one of two opposingsurfaces, the other surface being adapted to be urged against saidcircuit surface; said electro-optical material further having one endadapted to receive polarized light along an axis parallel to saidopposing surfaces, and another end opposite said first adapted forfurnishing said polarized light to a plurality of receivers.

Another embodiment for achieving this and other objects comprises alayer of an electro-optical material having a reflective coating upontwo opposing surfaces, one of said coated surfaces being semitransparentto incident polarized light, and the other of said coated surface beinghighly reflective and adapted to be applied to said circuit surface.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, where like reference numerals indicate like parts;

FIG. 1 represents an embodiment of ATE in accordance with the invention;

FIG. 2 represents parts of the equipment of FIG. 1;

FIG. 3 represents an optical assembly;

FIG. 4 represents an interface element;

FIG. 5 represents an alternative interface element;

FIG. 6 represents alternative ATE in accordance with the invention;

FIG. 7 represents a characteristic of the electro-optical effect withrespect to applied electrical voltage;

FIG. 8 is a perspective view representing the elements of the ATE ofFIG. 6;

FIG. 9 represents schematically an embodiment of a deflection systemused to optically examine the surface of the tested circuit board;

FIG. 10 represents an embodiment of an optical separator;

FIG. 11 shows placement of contact elements during a test;

FIGS. 12a and 12b show the anisotropy index of an electro-optical film;

FIG. 13 represents a sectional view of a printed circuit board on whichan electro-optical polymer has been incorporated for testing;

FIG. 13b shows in some detail an alternative to FIG. 13;

FIG. 14 shows an embodiment in which a performed electro-optical film isplaced upon a board;

FIG. 15 shows a plan view of the film used in FIG. 14;

FIG. 16 shows an alternative embodiment of the film used in FIG. 14;

FIGS. 17 and 18 show schematically a method of preparing a polymer filmuseful for putting the invention into practice;

FIG. 19 represents an arrangement permitting the detection of thePockels effect by an interferometic Fabry-Perot technique;

FIG. 20 shows an alternative to the technique of FIG. 19; and

FIG. 21 represents an arrangement permitting a detailed analysis of waveforms of the signals at circuit nodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention automatic test equipment includes(FIG. 1) a layer 10 of electro-optical medium of dimensionssubstantially equal to those of an assembled circuit, such as a printedcircuit board 11 to be tested, the assembly being combined such that theelectro-optical medium is electrically proximate the surface 12 of theboard 11 which carries the electric circuit, the latter comprisingcomponents such as 15. The surface 12 carries conductors such as 16 and17 which interconnect the components, and constitute the nodes of thecircuit. The medium 10 may be placed electrically proximate the surface12 by the intermediary of an interface element 18 which serves totransfer to its upper surface 100 (in the arrangement of FIG. 1) theelectrical potentials that appear at its lower surface 19.

In use, the lower surface 19 is in contact with the printed circuitboard, while the upper surface 100 is in contact with the medium 10.

Light from a source 101 may be directed at and received from any regionof the board surface 12 via positioning means 102, received light beingdirected toward a detector 103 by virtue of a beam splitter 104 such asa semi-transparent plate.

The detector 103 is sensitive to changes in the optical characteristicsof the medium 10 which are induced by electric field variations in theneighbourhood of the region of incidence of light on the medium 10, insuch a way as to produce an analogue of this field, adopting the form ofan electrical signal at the output 105 of this detector, this signaltherefore being also representative of any electrical signal which ispresent on the conductor the potential of which has been transmitted tothe electro-optical medium, in the region of incidence of the light, bythe interface member 18.

The board 11 has lengthwise connectors, such as 106 and 107 which areprinted on the board and which constitute the external nodes of thecircuit, with which it is possible to establish a physical connectionvia appropriate female connectors of standard type, the use of which iswell established in the art. A support or fixture 93, shownschematically, allows manipulation and/or support of the board during atest.

At test pattern may be supplied to exercise the circuit via theseexternal nodes, sufficient to cause a response at the node currentlybeing examined electro-optically, producing an analogue of the responseat output 105.

Based upon an analysis of the circuit to be tested, as is the practicein the ATE art, the output expected for the applied test pattern may bepredicted in advance, and the prediction 108 used as a basis forcomparison, in a comparator 109, with the obtained output 105. If theresult of the comparison is that the produced and predicted responsesdiffer, comparator 109 provides an indication of a test failure at itsoutput 110.

It will be appreciated that FIG. 1 is a sectional view taken through acircuit carrying board. To assist the clarity of description, a planview of the same board, with its conductors (hereinafter called tracks)and an electrical component is shown (FIG. 2), FIG. 1 corresponding to asection being taken at II--II' of FIG. 2. Typically, a circuit boardmight be 300 mm by 200 mm or more, the electro-optical medium 10 and therelaying member 18 having substantially the same dimensions. Theelectronic devices such as 15, which are mounted on the board often aredigital integrated circuits in dual inline P-dip packages; one part ofthe track network interconnecting these components is shown in FIG. 2;to the extent that the components are inserted in the lower face ofboard 11 (in the arrangement of FIG. 1), the component 15, not directlyvisible, is shown in ghosted marking in FIG. 2, with the exception ofits connection pins which, spanning the board to be soldered to thetracks carried by the surface 12 are shown directly in FIG. 2.

The solder pads between the ends of the pins of the components and thetracks constitute important discontinuities in the surface of the boardconductor carrying surface. In finished boards, this surface iscompletely covered with a protective lacquer which forms an intermediatelayer of some micro-meters thickness which is between the boardconductors and an external test probe. Moreover, these conductors areoften in practice covered with oxide which forms during manufacture andstorage. This oxide itself is a substance whose existence must be takeninto consideration in realising industrial testers.

Lastly, frequently modern printed circuit boards also carry componentson the face carrying the conductive track network. Nowadays boards carrytracks and components on both faces.

In use, the interface member 18 is placed in contact with the surface 12carrying the conductors, as partially shown in FIG. 2. This interfacemember 18 includes a plurality of conductive columns, such as column111, arranged in a matrix such that the potential of each conductor maybe relayed to the electro-optical medium 10 (also shown in part), via atleast one of the conductive columns. The nature and operation of theinterface member 18 will be described later.

It will be noted that each of the columns corresponds to a separatelyobservable region of the circuit and that light may be directed at eachof them, and received by reflection from each of them. The light isconstituted by a polarised light beam from a laser, and the analysis ofthe reflected light received consists of detection of the rotation ofthe plane of polarisation, for example by means of a polarimetricassembly including a Wollaston prism separating received light into twodetectable beams. The detector in fact comprises two detectors, so as topermit measurement of the difference in intensity between the two beams,and compensation of intensity variations in the source.

The electro-optical medium may be constituted by a crystal. In analternative, the electro-optical material may be constituted by apolymer film endowed with electro-optical properties. This film may beused in place of an electro-optical crystal layer in the ATE of FIG. 1.It may also be preformed to fit between components of the board to betested in direct contact with its conductors without recourse to aninterface member or only such a member of reduced thickness. As analternative, the film may be placed directly on the conductor carryingsurface before mounting to the components and of which the face incontact with the conductors has, intrinsically or from a serigraphicdeposition of metallic micro-particles, a degree of light reflectance,for example, the film may be integrated with the printed circuit boardas it is made. Such a structure, which allows the interface member to bedispensed with, leads to the making of electronic boards able, by virtueof their fabrication, to be subject to a test benefiting from all theadvantages of the invention.

The mechanism by which signals may be detected by an electro-opticaleffect will now be described in more detail (FIG. 3), more particularlywith reference to an embodiment of the invention making use of aninterface member. The essential characteristic described being howeverthe same in the case of use of a polymer film.

Output light from a laser 30 (for example an HeNe laser of wavelength(633 mm) is linearly polarised by a polariser 31 and concentrated, by alens 32, onto an acousto-optical deflector 33, so as to be directed, viaa lens 34, toward a point of inspection 35 of a reflective surface 36 ofan electro-optical crystal 37. Generally a medium power continuous laseris used, typically 5 to 100 mw.

An acousto-optical deflector 33, known in itself, is controlled by avoltage signal continuously output by a control device of the ATE (notshown), so as to deflect its received light to any point on theelectro-optical medium, examination of which is desired.

In a preferred embodiment, the deflector 33 comprises twoacousto-optical deflectors placed in series. A first control signalvoltage controls beam deflection along a first direction, correspondingfor example to one of the dimensions of the board to be tested, and theother voltage signal controls deflection along a second direction,preferably perpendicular to the first and corresponding for example tothe other board dimensions. Such use of two deflectors allows reductionin the time required to move from one monitoring point 35 to another toa time of the order of some microseconds at most.

The reflective surface 36 has the characteristic that an electricalpotential applied to one point on the surface is not propagated toothers. Thus, if the reflective qualities of this surface are obtainedby a metallic deposition, intrinsically a conductor, the layer would notbe continuously metallic but rather comprise a matrix of isolatedreflective particles, each of which is however connected to the regionof a point to be tested, such as a test point 38 of the board 39, viainterface number 300 (directly in the case of a polymer film). Inaccordance with one possible embodiment, the reflective surface 36 maybe formed of an intrinsically isolating material, for example a thinlayer of a dielectric material such as an alternating structure oflayers of titanium oxide TiO2 and silicon oxide SiO2.

A transparent electrode 301, for example constituted by a deposition ofgold or aluminum is deposited as a layer on the first face (that struckby the incident light) of the electro-optical crystal 37, and thiselectrode is kept at ground potential as a reference; thus, if theelectrical potential of the test point under inspection differs formthis reference, the polarisation of the reflected light is, by reason ofthe electric field applied across the thickness of the electro-opticalcrystal 37, different to that of the incident light, and this differenceis detected.

For a cubic crystal structure, such as Bismuth Germanium Oxide Bi4 Ge3012, the crystal axis (100) of which is optically orientated normal to aquarter wave plate 302, the phase shift of the light due the sustainedelectro-optical effect is proportional to the potential differencebetween the faces of the crystal 37 at the monitoring point andindependent of the electric field distribution in the crystal.

The reflected light is reflected toward the quarter wave plate 302 via asplitter 303; downstream of the plate 302, the light is directed, by alens 304, towards a Wollaston analyser 305, from which stem two beamsreaching respective photo-electric detectors 308 and 309 which producerespective electrical signals I1 and I2 at their outputs 306 and 307.

This type of polarimetric detection gives the following relationships:

    I1=I.(1+m) and

    I2=I.(1-m),

wherein I is an intensity proportional to the luminous intensity of thelaser 30 and in which m is the phase lag detected, expressed in radiansand implicitly small.

A differential amplifier 310 receiving the signals of outputs 306 and307 provides at its output 311 a signal I1-I2, thereby equal to 2Im. Inthe case where, during a test, the overall luminous intensity variationsof the laser 30 correspond to frequencies very much lower than thefrequency of variation of m, the value m may be directly obtained byadequate filtering of the electrical signal available at the output 311of the amplifier 310. As an alternative, a signal I1+I2 may be used toregulate the luminous intensity of the source.

For a crystal of non-cubic structure, such as that of Lithium NobiliumOxide Li N6 03, the electro-optical effect is also proportional to thepotential difference between the faces of the electro-optical crystal.The use of such a crystal requires some care in the case where thecoefficient of proportionality between the potential difference and theeffect obtained depends upon the orientation of the crystal cut.

The desirable properties of the electro-optical crystal are a lowabsorption, a low diffusion, and a low circular birefringence and goodlinearity. Where a highly birefringent material is used, such as LithiumNobilium Oxide for example, a small variation in the angle of incidenceof the light leads to a variation sensitive to static phase. Thermalchanges causing crystal thickness variations have similar effects.Preferably, both variations are avoided.

To improve the performance of a highly birefringent material, control ofthe polarisation of the incident light may, in accordance with thepresent invention, be provided, for example in response to angle ofincidence or temperature. Additionally or alternatively, the apparatusof the invention may include, relative to the electro-optical medium, acrystal structure presenting perpendicular orientations, but thicknessessubstantially identical, in such a way as to attenuate or cancel thebirefringence.

The electro-optical medium advantageously presents a resistivity atleast of the order of 10 ohm.cm, especially for testing low frequencycircuits, and a dielectric constant at most of the order of 100 so as tointroduce only a low capacitance (1 pf or less).

Internal to the interface member 18 (FIG. 4), a conductive column 111 issunk into an isolating flexible substrate 112. Each column, such ascolumn 111, is for example cylindrical. The substrate 112 contains anassembly of other mutually parallel columns each spaced lengthwise andextending widthwise along the interface member 18.

In use, this member is placed in contact with the conductor bearingsurface 12 of a printed circuit board 11 such that a lower surface 113of the column 111 is proximate a conductor 114 on the surface 12;surfaces of other columns similarly being proximate other conductors.The electrical potential in the conductor of column 111, may bemonitored at its upper surface 115.

A conductive film 116 is applied to upper surface 117, excepting theregions of the upper surfaces of the conductive columns, which may beearthed to provide a sharp step in potential at the upper surfaces ofthe columns for examination of the electro-optical effect as hereinbefore described.

It will be appreciated that since no current is required to pass alongthe conductive column 111, a high resistance can be tolerated in theregion of proximity to the conductor 114. Thus, the apparatus may beused to examine a circuit mounted on a board, such as board 11, to whicha film 118 of an insulating protective lacquer has been applied. This isa particularly important advantage of ATE in accordance with the presentinvention. It allows boards to be tested after all production steps,including lacquer application, have been completed. With prior artdevice testers lacquer cannot be applied as the nails have to makecontact with the conductors directly; even with functional testing,lacquer must be penetrated if the manual probe is to be used. With thepresent invention, boards in finished condition may be tested.

When the conductor bearing surface of the assembled circuit to be testedis uneven, as is the case for the board 5 represented in FIG. 5, thecorresponding surface in the interface member is profiled, moulded orfretted to a suitable shape. The board 50 has for example a circuitusing surface mounted components, (such as 52) the connection pins ofwhich are directly affixed to the conductive tracks 53 withoutpenetrating the board 50. In such an arrangement, a plane conductivesurface may not be present, thus the interface member 51 is adapted toreceive the surface mount components, to maintain a plane surfaceproximate the electro-optical medium 54.

In an embodiment of a tester in accordance with the inventionrepresented in FIG. 9, a mechanical deflection as well as anacousto-optical deflection is used to extend the scanned circuitsurface. A laser 430 emits the test beam at an acousto-opticaldeflection apparatus 433 via a polariser 431 and a lens 432. Theapparatus 433 comprises two acousto-optical deflectors for example ofthe types made by AUTOMATES ET AUTOMATISMES, 19, rue de Paris 78460CHEVREUSE France.

The first deflector is mounted inside a housing 433 to deflect the laserbeam in a first direction a and the second deflector deflects the beamso deviated in a perpendicular direction b, such that the two deflectorcombination allows the sweeping of a square surface of 50×50 millimetersat a distance of 1000 millimeters from the output of the lens 432(corresponding to its focal length).

The beam emerging from the apparatus 433 falls on a pivoting mirror 441of a first mechanical deflector 443 which reflects it onto a pivotingmirror 442 of a second mechanical deflector 444. The combination of thedisplacements of deflectors 443 and 444 in two directions A and Bparallel to a and b allows the emergent beam 445 to sweep a rectangularsurface of 500×500 millimeters in the focal plane of the lens. The beam445 falls on the convex face 449 of a plano-convex lens of which thedimensions are 500×500 millimeters, made for example in BK7 glass.

A rectangular layer of a mosaic of BGO (Bismuth Germanium Oxide, Bi4 G33012) crystals is affixed to the plane face of the lens. The dimensionsof the lens 450 and the electro-optical crystal layer 451 substantiallycorrespond to the total area to be swept by the beam 445 exiting thesystem of deflectors 443, 444.

The BGO layer 451 has a thickness in the region of one (1) millimeter.In this example, the mating of the BGO layer 451 to the plane face ofthe lens 450 allows mechanical vibrations which occur naturally in thecrystal as a result of piezo-electric resonance phenomena to beannulled. Such vibrations would be evident by virtue of the parasiticoptical signals that they would cause by photo-elastic effects in thecrystal.

The laser 430, polariser 431, acousto-optic and mechanical deflectors433, 443, 444 and also lens 450 with the BGO layers 451, assembly isintegrated with the optical recovery and detection system for thereflected light by the layer 451. This assembly, not illustrated in FIG.9, is shown as 319 in FIG. 3 and comprises the splitter 303, the quarterwave plate 302 on lens 304, the WOLLASTON Prism 305, both photoelectricdetectors 308 and 309 and the differential amplifier 310 forming anoptical probe incorporated in the ATE head.

In use, an interface member 462 analogous to the interface member 18 (ofFIG. 1) having anisotropic conductive properties is firstly placedagainst a face to be tested of a printed circuit board 460 (FIG. 9) toform an analogue of the card 460 conductor voltages in contact with oneof its faces at its other face 463. This face 463 and the free face ofthe electro-optical crystal 451 at the heart of the optical probe are incontact for the test and firmly urged together by pressure means notrepresented allowing the elimination or minimising of all parasiticspacing between the contacting faces of the BGO layer 451, the interfacemember 462 and the board to be tested. In this example an elastomersheet made by JSC TECHNIC in the Federal Republic of Germany and soldunder the name `ZEBRA bidimensionnel` has been used for the interfacemember; its thickness being between 0.1 and 5 millimeters for example inaccordance with the board type and its surface discontinuities.

In FIG. 9, the checker region 470 corresponds to an elementary surfaceof acousto-optical scanning of the crystal of 50×50 mm. By virtue of adisplacement by the mechanical deflection system, acousto-opticalscanning of one hundred (100) such elementary surfaces next to eachother over the surface of the electro-optical crystal 451 is possible.

Typically, the acousto-optical scanning rate which may be obtained is100 KHz (the frequency of getting form one test point to another). Themechanical scan allows a change form one elementary region 470 toanother in 50 milliseconds or so.

Consider a rectilinearly polarised light beam 445 at the input of thecrystal layer 451 (FIG. 9). Its polarisation remains unchanged whilecrossing the layer if the conductor of the board to be tested in theregion of the point of incidence of the beam has no voltage load. Theapplication of a voltage creates a phase shift p(u) between the twocomponents of the electric field of the light. The state of beampolarisation which passes back through the lens 450 at the output of theBGO layer 451 is then elliptical.

The emergent beam is redirected by the splitter 303 (FIG. 3) toward adetection lens 304, then is divided into two components by the WollastonPrism 305. The intensities of these components are represented by

    I1=1/2Io (1+cos (p+Po))

    I2=1/2Io (1-cos (p+Po))

in which Io is the incidence intensity and Po is the static phase lagintroduced by the quarter wave plate 302.

The differential amplifier 310 gives a signal S

    S=I1-I2=Io (P+Po)

If the phase shift p is zero, the signal s is negligible in many casesdue to the small amount of polarisation variation produced in the layer451.

By way of example, for a BGO crystal of index 2 and of electro-opticalcoefficient equal to 1 pm/v, for a laser wavelength of 647 mm (Kryptonlaser) a value of P=8.10-4 rad/v. is obtained.

In response, with a quarter wave plate, Po=pi/2 and

    S=Io×P

the signal S varies linearly with V and values of P of the order of 10-4radians may be straightforwardly detected. By virtue of the differentialmethod adopted, this is true even if laser source intensity is subjectto relatively slow fluctuations in time (up to about 10 per cent).

The deformations caused by fixing the BGO crystals may give rise toparasitic birefingences which are different from one point to another.As a result Po varies from one point to another. P1 is biased to returnto P1/2 after each positioning of the beam and before electricalmeasurements by inserting a Kerr cell 480 between the quarter wave plate302 and the Wollaston Prism. This apparatus is constituted by a plate ofelectro-optical material placed between two transparent electrodes in avariable electric field supplied by a feedback loop 482 comprising aswitch 484 and a variable amplifier 486 from the output 311 of thedifferential amplifier 310. The phase shift introduced by the plate 480between the beam field components which pass through it then makesadjustment to the level required to cancel the background component ofthe signal S corresponding to the point tested. With test signals of asuitably high frequency, the high frequency component of the signal Scontains the desired test information.

The modulation depth of the arrangement may be further improved byintroducing an imperfect polariser 340 (FIG. 10) between the crystallayer output and the quarter wave plate 302. Polariser 340 is a cubeintercepting the beam 335 leaving the lens 34 and used in place ofsplitter 303. Assume that a system of axes 350 represents thepolarisation of light emerging from the lens, P being the angle ofpolarisation with respect to the initial direction of linearpolarisation along the Y axis. The x and y components of sector 352,representing the polarised wave of the beam 335, represent the majoraxes of the elipse of polarisation, the Wollaston Prism 305 (FIG. 3)being orientated so as to split the light components along these twoaxes.

The splitter cube 340 separates the beam 335 into two beams, onetransmitted 334, the other reflected 336. The interface 355 of the cubeis arranged such that it behaves as a bad polariser for the reflectedlight and increases its ellipticity of polarisation. The component alongthe Y axis is greatly reduced as against the X component as is shown bythe diagram 360 of FIG. 10 and in accordance with the principlesdiscussed in French patent application having the number 88/04177 filedMar. 30, 1988 in the name of the applicant. By contrast for thetransmitted portion of the beam 334 this is linearly polarised along Y(diagram 370).

The components I1 and I2 received after separation of the beam 336 bythe Wollaston Prism are expressed as follows:

    I1=(I+Ap) Io/2A2

    I2=(I+Ap) Io/2A2

A is a coefficient greater than 1 given by the characteristics of thepolariser cube 340. These relationships show that a more powerful lasermay be used to increase the modulated depth without saturating thedetectors at the output of the Wollaston Prism.

The interfaces between the printed circuit board 460, the interfacemember 462 and the crystal layer 451 have been shown schematically inFIG. 11. Two conductors 467 and 468 forming two test points areindicated one under a voltage V1, the other at ground potential. Theconductors are often covered in oxide. When the board is completed, theyare covered with a lacquer. The layers of oxide or lacquer form aspacing 469 between the upper face of the board and the opposing face ofthe interface member 462.

An electrical equivalent circuit diagram of the arrangement is shown inFIG. 11. By virtue of the capacitances of the crystal, of the conductorand of the spacing 469, the voltage V2 which reaches the crystal inresponse to V1 is all the weaker as the capacitance of the spacing 469is lower and that of the crystal is greater. Moreover, for a giveninterface member 462, the thickness of which is determined by thecomponents mounted on the surface of the card 460, the more the interval469 is made larger and the more the crosstalk, represented by the ratioV3/V2 is increased.

In one example, the parameters were as follows:

    ______________________________________                                        thickness of the interface member 462                                                                e =    2 mm                                            thickness of the BGO crystal 451                                                                     =      1 mm                                            dielectric constant of the crystal                                                                   e' =   16                                              distance between the two test points                                                                 d =    400 micros                                      ______________________________________                                    

If the maximum allowable crosstalk is about 10%, the height h of thespacing 469 must not exceed 1.5 microns. It is therefore desirable todeposit an electro-optical material having a dielectric constant as lowas possible which at the same time presents an electro-opticalcoefficient as high as possible.

The relationship between the electro-optical sensitivity index of thecrystal n³ r (n being the refractive index of the medium and r itselectro-optical coefficient) and the dielectric constant e' constitutesa yardstick (figure of merit) for the selection of material for use asthe electro-optical layer 451. Table 1 below gives an indication of thisrelationship for different crystals.

                  TABLE 1                                                         ______________________________________                                                               Avail Dim                                              Crystal    n3r/e'      (cm2)     Origin                                       ______________________________________                                        MNA        50          1         France                                       ZnTe       10.8        1         USA                                          AsGa       3           100       France                                       CuCl       1.9         1         France                                       Li Nb 03   1.6         10        USA                                          Bi12 Si 020                                                                              1.2         12        Japan                                        Bi4 Ge3 012                                                                              0.5         100       France                                       K Nb 03    0.5         2         France                                       ______________________________________                                    

With the apparatus with the BGO layer as envisaged above, reliabledetection of test bits of 1 volt amplitude at a frequency of 5 MHzapplied to test points 2 mm apart and separated by a ground conductorhas been obtained. The thickness of the elastomer interface member was 2mm, the contact surfaces without oxide and covered by a lacquer layer of5 microns thickness.

Consideration of the preceding table shows that an organic compositionsuch as MNA (2 methyl -4--nitroaniline) shows an increased figure ofmerit making this type of substance most interesting for the envisagedapplications. Such a substance may be used not only in crystalline form,but also in a form combined with a support material in which themolecules of such an active electro-optical substance are incorporated.As a support material perspex (PMMA--poly methyl methacrylate) may thusbe used with a density of MNA about 15%. The MNA is therefore then usedas a dopant having molecules which are retained in the PMMA matrix, tomake the composition electro-optical. Other possible dopants possessingelectro-optical properties are for example the following:

    DAN [4- (N,N-dimethylamino)-3-acetamidomitrobenzene]

    COANP [2-cyclo-octylamino -5- nitropyridine]

    PAN [4-N-pyrrolydino -3- acetaminomitrobenzene]

    MBANP [2-(alpha-methylbenzylanino)-5-nitropyridine]

This type of polymer substance is the topic of many developmentprogrammes currently being undertaken by several enterprises,universities and research centres in the field such as the LockheedMissiles and Space Company, Inc., Hoechst Celaneses Corporation andother large houses in the chemical field. (See for example theconference proceedings of the Symposium Entitled NATO Advanced Workshop:"Polymers for non linear optics", Sophia Antipolis Jun. 19-24, 1988.)The polymers obtained may be used to form films, fibres or thin layersor thicknesses on a known substrate.

A particularly interesting characteristic of these polymers is thepotential to use them in the form of films capable of being produced inlarge quantities and at reasonable cost. These films may be used inthicknesses of some microns (for example 10 microns) on transparentsupports such as glass. They may also be delivered directly in the formof resilient films made up of several adjacent layers of elementaryfilms (for example 500 microns thick). After drying, the molecules ofthe electro-optically active ingredient are captured in a supportmaterial in the amorphous state without particular orientation. To makethe material electro-optic it is necessary to heat to a temperaturesufficient to allow the active molecules to regain a certain mobilityinside the matrix. The value of this transition temperature may varywith the polymer but may typically be around 100° to 120° C. In thisstate they may be subjected to orientation with respect to the supportunder the effect of an electric field. The molecules tend to orientatethemselves in the direction of the exciting field. The stronger theexciting field, the higher the proportion of active molecules orientatedin the direction of the field. When the temperature is again loweredwhilst the molecules are oriented under the effect of the field, theykeep this orientation. The material thus keeps an orientated structurewhich may be shown by an anisotropic optical behaviour (Pockels effect)in the presence of an electric field. Thus when the material is struckby linearly polarised incident light in the absence of an electric fieldthe material does not give any change in polarisation. By contrast, inthe presence of an electric field, the transmitted light sustains anelliptical polarisation linearly related to the intensity of the appliedfield.

FIG. 12a shows the index distribution in an electro-optical film 500realised in a polymer of the type previously described, the molecules ofwhich have been orientated by the application of an electric field,called an aligning field, in a direction normal to its surface whilstthe temperature was lowered below the transition region beyond which theelectro-optical dopant molecules loose their mobility. An ellipsoidrepresented as 502 describes the refractive index distribution of thematerial whilst an electric field (called a detecting field)perpendicular to its plane is applied. This ellipsoid shows the spatialrefractive index variations of the material. It has a symmetry ofrevolution with respect to the normal 504 in the plane of the film 500which reflects the fact that the material is optically isotropicparallel to the plane of the film.

If the film 500 is illuminated with a polarised incident beam 506 normalto the film, the light recovered from the film (by transmission orfollowing reflection from the opposite face 508 of the film) will notshow a changed state of polarisation. This explains that when such afilm is used for example in place of the BGO layer 451 of FIG. 9voltages at the scanned circuit node cannot be detected. On the otherhand an incident beam 510 angled with respect to the plane of the filmwill have its state of polarisation altered.

It is necessary therefore in this case to use a polarised incident lightbeam angled with respect to the electo-optical layer in order to revealthe Pockels effect created by the voltages in the tested circuit.

In FIG. 12b the ellipsoid of indices obtained with the same materialunder the action of a detecting electric field, normal to the plane ofthe film, is shown when the molecules of the film 500 have beenpreviously orientated in the plane of the film by an aligning electricfield. In this case the ellipsoid of the indices 512 has a symmetry ofrevolution with respect to an axis 513 in the plane of the film 500. Anormal incident beam 506 then has its polarisation changed as a functionof the difference in indices r2 and r3 along the principal axes of theellipse in the plane of the film. Under these conditions the assembly ofFIG. 9 (that is with normal incidence) with a polarimetric analysisapparatus such as represented in FIG. 3 allows the exploitation of thePockels effect created by the circuit to be tested in the polymer filmincorporated in the optical probe or the board.

An apparatus for orientating the molecules of the active composition inthe plane of the polymer film is illustrated in FIGS. 17 and 18schematically. A strip of polymer 800 is placed in a tunnel oven 801heated to a temperature T of about 100° C. (slightly higher than thetransition temperature described above.)

The strip is guided in the interface between two opposing metal plates802 and 804 which are maintained at a continuous identical electricpotential+V. At the output 805 of the tunnel oven, the plate enters asecond tunnel 810 defined by two opposing metal plates 812 and 814 whichare maintained for example at zero potential. The result is that thespacing 815 between the heating tunnel exit 801 and the second tunnel810 is subject to an electric field substantially uniform and parallelin the plane of the strip 800 which lays in this space 815. Theelectro-optically active molecules of the film tend to orientatethemselves parallel to this direction under the local effect of thefield at the exit 805 of the tunnel 801. Two nozzles 818 and 819 toeither side of the film 800 open in the space 815 where they deliver aflow 820 of inert gas (for example argon or a sulphur hexafloride SF6)cooled to -40° C. to both faces of the film 800. The gas is exhaustedfrom between the plates of the second tunnel by means not shown. Thetemperature of the film 800 traverses the transition region within thespace 815. The active molecules maintain a preferred orientation in theplane of the film which cools in the second tunnel.

The preceding data concerning the orientation of the film structure andthe incidence of the test light permits use of the electro-opticalpolymer film 500 for the testing of the signals in a circuit by thepolarimetric method.

Alternatively, and in accordance with another aspect of the invention, aFabry-Perot interferometric method may be adopted in place ofpolarimetry to detect the Pockels effect under the influence of thevoltages to be tested.

In FIG. 19 part of a circuit board 840, typical of an assembled circuit,is shown comprising a support 841 of isolating material on a face ofwhich several electronic components are mounted; these areinterconnected by conductive links such a 848.

A layer or plate of electo-optical material 842 is placed proximate thecircuit supporting board 841 in contact with the upper surface of theconductor 848. It receives, given normal incidence, a laser beam emittedby a laser source 844. The surface of the film 842 facing the lasersource is covered with a layer 845 of conductive and semi-transparantmaterial which forms a semi-transparant mirror of average valuecoefficient of reflection. The other face of the film 842 (opposite thecircuit board to be tested) is covered with a layer 846 of anelectrically conductive and high coefficient of reflection material, forexample a layer of aluminum thicker than the layer 845 so as to reflecta substantial part of the light which reaches it, the remainder beingabsorbed. A layer 847 is provided to absorb that part of the lighttransmitted by the reflecting face 846. The layer 846 may be formed ofparticles or fragments isolated one from another to avoid creating shortcircuits between neighbouring conductive tracks on the board to betested 840.

Interference is established between the light reflected by the parallelmirrors 845 and 847; the light beam emanating from the film (shown inthe Figure with a double arrow) is defected by a semi-transparent mirror851 and superimposed on the common path of the emitted beam 843 and theoutput beam 849 toward optical analysis means 850 for measuring theintensity of the light signal 849. As is known, the interferencephenomenon is a function of the distance separating the mirrors, that isthe thickness (e) of the film, the wavelength (lambda) and therefractive index (a) of the material of the film. The index "n" varieswith the electric field to which the film is subject, itself a functionof the potential V which it is desired to measure.

Therefore, measurement of the luminous intensity of the signal at thereceiver 850 allows a measurement of the potential V at a given point onthe circuit 800 to be obtained. It will be noted in particular that ifthe layer of electro-optical material 842 is a polymer film of the typedescribed above (and which could be integrated with the board 840) themeasurement of the Pockels effect is possible with perpendicularincidence with this interferometric technique, even when theelectro-optical molecules of the film have been originally orientatedperpendicular to its plane.

In order to alleviated the consequences of thickness differences in theelectro-optical film or layer, the light source is adapted to emit avariable wavelength light beam. This means that the operating point ofthe apparatus may be shifted on its intensity characteristic curve (FIG.20), of intensity as a function of PH1=[2pi/lamba]n.e from a point Asituated on a first part of the curve where I=Imax whatever PHI(therefore of zero sensitivity) to a point B situated on a secondportion of the curve where I (being between Imax and O) varies greatlyas a function of PHI; the sensitivity is a function of the variation inI on the second portion of the curve about the point B; preferably pointB corresponds to about Imax/2. When point B is fixed, the measurementsare then affected by the wavelength corresponding to the point B.

In FIG. 13, a printed circuit board 600 includes a substrate 602 of amaterial traditionally composed of epoxy and glass fibre which compriseson each of its faces 603 and 604 metallisations or conductive trackssuch as 605 and 606, forming a network to which the components mountedon the board are connected.

The face 604 of the board is covered with a layer of polymerelectro-optical film 608 which extends over substantially all thesurface above the metallic tracks 606 which are formed there. The film608 is incorporated on the card after the formation of themetallisations, but before the placement of components, for example bybonding. One side of the file 608, face 604, is covered with achecker-board pattern of elementary mirrors by means of a layer ofreflecting aluminum 609. The size of the particles or pieces of thepattern is such that each piece cannot give rise to a short circuitbetween two neighbouring metallic tracks. On the opposing face, the film608 is covered with another layer of aluminum 610 of a thicknesssufficiently low to be transparent to test light projected at the boardfrom direction 612. The layer 610 constitutes a reference electrode forthe electric fields generated in the thickness of the film 608 by thevoltages applied to the conductors 606.

The board 600 carries components such as discrete element 614 orintegrated circuit package 616 on its surface 603. The components haveconnection pins 618 which penetrate the board substrate in holes 620placed at right angles to the metallic tracks 606 on the other surfaceof the board, to which they are connected via solder pads 622. Alsoshown is a components 624 mounted on the surface 604 of the board, itstwo outer faces 625 and 626 being connected to two metallic tracks 606respectively by two beads of solder 628. Before assembly of thecomponents 614, 616 and 624 openings such as 630 have been formed in thepolymer film 608 around the expected solder beads 622 and 628 to avoidany electrical contact between the metallic solder and the referenceelectrode 610. These openings may be formed by grinding before placementof the components on the card or be made in the film 608 before itsjoining to the surface 604 of the card. Alternatively, a film or filmelements, preformed as a function of the board regions carrying theconductors to be tested may be placed upon the board before mounting.Contact regions such as 632 between the upper surface of themetallisations 606 and the film 608 provide the test points.

The test points are monitored by projecting an incident laser beam andanalysing the beam reflected by the corresponding metallic mirror 609.This analysis is made with a test probe analogous to the apparatus ofFIG. 9, except that the BGO layer 451 and the interface layer 462 areomitted. In fact, the light leaving the plano-convex lens 450 strikesthe card mounted adjacent its plane face directly. The beam returned byeach point of the tested board 600, changed by the effect of the voltagedriving this point, is detected and analysed by an arrangement such asthat 319 of FIG. 3. The closeness of contact between film 608 and thetested conductor 606 ensures excellent optical conservation of thesignals to be monitored and good spatial resolution. In certain boardarrangements, (FIG. 13b) wherein the conductive regions 606A to beexamined are by construction placed proximate a ground connected region606B, an electric field parallel to the plane of the electro-opticalfilm is established (field lines 640, 641 FIG. 13b). The existence ofthis field may be directly tested without any opposite polarityelectrode that might otherwise be necessary. The Pockels effect thenappears by virtue of a substantially parallel field in the plane of thefilm, rather than a perpendicular field.

In accordance with another embodiment of the invention, use of aelectro-optical polymer film directly in a tester optical probe isenvisaged. In fact, the good characteristics of these materials, whatwith the high level of their electro-optical coefficient and their lowdielectric constant, make them well adapted to this application. Theymay be made in the form of a layer bonded, for example, on the planeface of the plano-convex lens 450 (FIG. 9) in place of the BGO crystallayer 451.

In accordance with another advantageous technique, the use for each typeof board of an electro-optical transducer specific to that type of boardis envisaged. In this connection the fact that the costs associated withpolymers of the type indicated are not very great is put to good effect.It is therefore possible to make for each new type of board to be testeda transducer designed to be associated with the optical probe during thetest but which is adapted to that form of board.

The transducer 700 (FIGS. 14 and 15) is in the form of a plate 702 ofglass or a transparent plastic material of low photo-elasticity andstraightforwardly workable so as to be able to create interior openingsor cavities in which the components, the solder pads and otherdiscontinuities in the upper surface of a printed circuit card 705 arelocated when the transducer 700 is coupled for the test.

An electro-optical polymer film of the type already described withreference to FIG. 12 is deposited onto the face 710 of the transducer700. Film 712 is covered by an aluminum transparent electrode 714 whereit mates with the support plate 702. The lower face of the film 712 iscovered with a reflective layer 716, also in aluminium. As previously,this layer is not continuous, but formed of particles mutually spacedapart such that a particle in contact with a conductor on the surface ofthe board cannot also contact or have its potential influenced by aneighbouring conductor.

The openings 720, 721, for example are arranged in the transducer 700 toallow components such as 724 to enter or beads of solder such as 725, bywhich the pins 726 of components 727 passing through the board 705 arefixed to conductors 728 to the surface 730 of the board against whichthe transducer 700 is placed, to be accommodated.

The plate 702 gives rigidity to the transducer sufficient to allow theelectro-optical polymer film 712 to be placed against the conductorssuch as 728 and 732 at the surface 730 of the board 705 when the board705 and the transducer 700 are brought together, by manipulationapparatus (not shown). The parasitic capacitances between the sensitivefilm 712 and the contacts are at a minimum level, which allows goodspatial resolution to be obtained, for example 0.1 mm, with a lacquer of10 microns thickness. The result is substantially better that theresolution possible with a relatively thick interface elastomer such asdescribed with reference to FIGS. 1 to 3. The reduced thickness of thislacquer allows crosstalk between very close test points to be kept tolow levels.

The support plate may be made by commonplace mechanical means. Thesensitive film (FIG. 15) may itself be made by laser. The transducerassembly 700 may be fixed in contact directly with (or immediatelyadjacent to) the planar face of the field lens (cf. 450, FIG. 9) of theoptical probe in place of the arrangement formed by the BGO lacquer 451and the elastomer interface 462. Alternatively it may be manipulatedseparately form this lens at the time of inserting the board to betested. As in the case of an electro-optical board of FIG. 13, the filmportions or electro-optically sensitive films may be separate andreduced to a predetermined number of test points or regions distributedover the board surface. Regions without mirrors or openings 727 areprovided in the transducer 700 to allow the positioning of referencemarks 728 from which the beam deflection system is calibrated toprecisely direct the incident beam toward the conductors for nodes to betested in the circuit. The deflection assembly 433, 443, 444 (FIG. 9)control system may then be programmed to selectively interrogate thenodes of the board corresponding to the regions of the transducer whichare provided with electro-optical film or material.

FIG. 16 shows schematically a sectional view of an electro-optical film740 provided on a transparent reference electrode 742 on one of itsfaces. Its other face is covered by a checker pattern of small mirrors744 formed by a thick deposition of aluminium. To further increase thecontrast of measurement, the polymer is etched (chemically for example)between the mirrors to create dips 706 of depth of the order of therequired resolution (0.1 mm for example). Thus the capacitance betweentwo neighbouring mirrors 744 is reduced. This film may be used in thetransducer 700 of FIGS. 14 and 15.

Over and above the face, already outlined, that the invention allowscompleted boards to be tested, that is boards covered with lacquer,another advantage of the invention is that the activity of internalnodes may be observed with a disruption all but negligible with respectto prior art testers. In fact, all contact with a probe, that is anelement in which current flows, disrupts the normal operation of themonitored circuit, such that it may, with prior art testers, not beoperating in the same way when it is tested and when it is not.

In conventional testers, the maximum speed at which a monitored circuitmay function may, in some cases, be limited by the requirements of thetest. Thus, tests are not conducted in the same operational environmentas that in which the circuit ought normally to function. Thisconstitutes a major problem with prior art testers, the manualdiagnostic probe of which has a high capacitance (of the order of 100pf). On the other hand, the interface member 18 may only have acapacitance of 1 pf, thus allowing circuit operation at full speedduring test.

The electro-optical material mentioned above produces a substantiallylinear electro-optical effect at the wavelengths envisaged (Pockelseffect), that is variation of angle of polarisation of a beam crossingthese materials under the effect of an electric field in the region ofcrossing in proportion to this field. Other materials present aquadratic effect (Kerr effect) in accordance with which the variation inthe angle of polarisation is proportional to the square of the electricfield. This property is put to use in an alternative embodiment of theinvention (FIG. 6), wherein the light is directed generally andlaterally at a quadratic electro-optic medium 60 placed electricallyproximate conductors, such as conductor 61, printed on a board 62 of anelectric circuit to be tested.

Light is received at an opposite edge face of medium 50 by a detector 63which produces an output 64 in accordance with the principles alreadyoutlined. Output 64 may be used by ATE as herein before described.

Electrodes, such as electrode 65, are deposited upon the upper surfaceof the electro-optical medium 60 along the path of the light. Electricalconnections (not shown) allow each electrode to be biased to a chosenpotential, for example to zero or to a potential of twenty voltage withrespect to a reference potential.

Under these conditions, the nature of the transmitted light received atdetector 63 will be dependent upon electrical potentials appearingacross the electro-optical medium. As the circuit below the medium isexercised these electrical potentials will be relayed to the undersideof the medium 60 via a relaying member 66. Taking as an example adigital circuit, having logic 0 at 0 volts and logic 1 at 5 volts andassuming that all biasing electrodes 65 are biassed to the 0 volts,either a zero or a 5 volt potential will be applied across theelectro-optical medium 60.

The characteristic (FIG. 7) of the electro-optical medium chosen forthis embodiment of the invention, for example a PLZT ceramic compositionof the type used for the control of optical gates on laser beams, is acurve 50, defining effect against voltage. Thus for 0 to 5 volts appliedpotential, the electro-optical effect will be detected between a and balong the ordinate. Assume now that one electrode, say electrode 65, isbiassed to a potential of 20 v. The effective potential appearing acrossthe medium 60 in the region of the conductor 61 is thus either 20 volts(logic 0) or 15 volts (logic 1). These potentials will produce an effectbetween c and d.

It will be appreciated that dynamic signals appearing in the region of abiassed electrode may therefore be discriminated from cumulative effectsproduced in the unbiassed region, the swing c-d being much greater thanthe swing a-b. Hence by selectively biassing the electrodes, activity inany region along the tract of the light may be examined.

For examining any region of activity, an array of detectors 80 (FIG. 8)extending edgewise is required. Where parts of the embodiment of FIG. 6correspond to those of FIG. 8, common reference numerals have been used.Biassing electrodes, such as electrode 65 extend laterally stripwiseacross the medium 60 in a direction substantially perpendicular to themean direction of light propagation in the medium, such that electricalactivity in any region may be examined by energising the electrode whichbiasses that region and by selecting the detector output signal, such as81, which receives the light which crosses that region.

The way in which ATE in accordance with the present invention may bearranged to operate will not be considered.

In an assembled circuit, such as a printed circuit board, the state ofeach of the outputs clearly depends upon the previous states of theinputs. For example, the state SK of the Kth output, for a known to begood circuit, is a function FK of the array [E] of the input states,this may be written:

    SK=FK([E]O).

The purpose of prior art functional testers is to verify that for anarray [E] of input states assigned to the circuit, the state of eachoutput, such as state SK of the Kth, is well founded on the array [E] ofinput states through application of a function, such as FK, whichcharacterizes correct operation of the circuit. If that is not the case,that is if the state of at least one output, such as the kth, is notcorrect and, for example, S'K takes the place of SK, the monitoredcircuit is clearly faulty.

This information, however, provides no help for the repair of thecircuit, it will also be appreciated that the dependence of SK and [E]is very complex.

Upon further investigation, it will be seen that the state of each ofthe outputs depends upon the state of the inputs via the intermediary ofthe states adopted by the internal circuit nodes. For a good circuit, itmay be shown that the state SK of the kth output depends, by virtue of afunction denoted as GK, on the array [E] of the input states, not onlydirectly, but through the intermediary of the state I1 of a firstinternal node, the state I2 of a second internal node, etc. . . . . Thismay be expressed, for the case of a circuit with n internal nodes, as:

    SK=GK (I1, I2, . . . , In)

If, instead of adopting the state SK, the Kth output adopts the stateS'K, which maybe because instead of adopting the state I1, the firstnode adopts a state I'1, and/or because instead of adopting the state I3the third node adopts a state I'3, etc. . . . , this gives rise to aseries of possibilities, such as:

    S'K=GK (I'1, I2, . . . , In) or

    S'K=GK (I1, I2, I'3, . . . In) or

    S'K=GK (I'1, I2, I'3, . . . In), etc.

In general, the appearance of an abnormal state S'K may be a priori dueto a large number of possible causes, amongst which a prior artfunctional tester cannot detect the actual cause without recourse tolong and difficult additional functions.

By contrast, ATE in accordance with the present invention may, duringdiagnosis, that is after a functional problem with the tested circuithas been established, shown by at least one abnormal result in theexamination of the circuit output nodes, be used as follows. First ofall an internal node is selected and examined, and all possible ordesirable state combinations are applied to the circuit input nodeswhilst the corresponding node states are examined and recorded. From theresults data obtained, as great a number as possible of possible faulthypotheses are eliminated. Then another node is chosen and the sameprocedure repeated. The internal nodes are thus examined one by oneuntil evidence of the cause of the fault is revealed.

Those skilled in the art will understand that the present invention isnot limited to the embodiments described and put forward above. Hence,the present invention is not limited to the technique of backtrackingwhich involves the use of expert systems. Those skilled in the art willalso understand that although the embodiments described make use of butone light beam, use of several beams is with the scope of the presentinvention.

Moreover, the arrangement which has just been described for circuittesting may be combined with a spectrum analyser of the signals providedby such a circuit. In the system described thus far the signals arerecorded in real time and the test takes account only of the presence orabsence of pulses at predetermined instants. The passband of the system,limited by that of the detectors used, may be in the region of 100 MHzfor example. A detailed analysis of waveforms at a much greaterfrequency may however be obtained by operating as described herein afterwith reference to FIG. 21, in accordance with a sampling (stroboscope)technique.

A CW laser of the type already described 901 is arranged to project thetest light toward an optical system 902. This system is of the typepreviously described. It produces and sweeps an analysis light beam 903projected in the direction of a circuit 907 to be tested across anelectro-optical arrangement comprising for example a field lens and anelectro-optical transducer of the type described with reference to FIG.9. The reflected optical signals are divided by a splitter andtransmitted to a detector 914.

A second laser 920 of the pulsed type is provided which emits at apredetermined repetition rate pulses which are very short with respectto the duration of signals produced at the circuit nodes in response totest excitations applied to the inputs 922 of the circuit 907. The lightpulses of laser 920 are directed on the one hand to a photodetector 924and on the other, via a splitter 925, across a variable optical delayline 926, toward a reflector 932 which directs the pulses toward theinput of the optical system 902 along the same axis as the light outputof the laser 900. The delay line 926 comprises two movable reflectors927 and 928 allowing two right angle bends in the direction of the beamoutput of the splitter 925, and a fixed reflector 929 to realign thebeam in the direction of reflector 932. The optical delay imposed on thebeam between the pulsed laser 920 and the optical system 902 may bemodulated by bringing together or moving apart the two reflectors 927and 928 and the fixed mirror 929 (arrow 930).

The electrical output of the photo detector 924 is connected tosynchronisation apparatus 940 which controls the test signal generator942 connected to the inputs 922 of the circuit board 907 such that theelectric pulses are applied to the inputs with a repetition rate equalto that of the pulsed laser.

The duration of the pulses of laser 920 is very short compared with thepulses at the inputs 22. The signal provided by the detection system914, of low bandwidth compared with the length of the laser pulses, isintegrated over a large number of samples. Each point of a waveform or acircuit node may be examined by a suitable adjustment of the opticalpath between the pulsed laser 920 and the optical system 902.Alternatively, an electrical adjustment of the synchronisation pulsesmay be made.

In use in testing the continuous laser 900 emits; the pulsed laser isoff. For a detailed timing analysis, the laser 900 is switched off andthe pulsed laser 920 excited.

We claim:
 1. An apparatus for testing a circuit having voltage-bearingelements disposed proximate a surface of a body which includes thecircuit, comprising:sensor means comprising:a layer of electro-opticalmaterial having parallel first and second opposing surfaces; anelectrode disposed on the second surface of the electro-opticalmaterial; and means for conforming to the body surface so that voltageson the voltage-bearing elements are imposed on the first surface of theelectro-optical material; means for applying test signals to thecircuit; means for establishing an electric field which traverses thelayer of electro-optical material by applying a potential to theelectrode with the conforming means in contact with the body surface sothat an optical property of the electro-optical material varies insubstantial synchronism with voltages occurring on the voltage-bearingelements in response to the test signals; means for measuring thevarying optical property in regions of the electro-optical material; andmeans responsive to the measuring means for determining a testparameter.
 2. An apparatus as in claim 1 wherein:the electro-opticalmaterial is an electro-optical crystal; the conforming means comprises aflexible coupling medium having a plurality of conductive channels, eachof the conductive channels being insulated from the others and extendingfrom one to the other of two opposing surfaces of the coupling medium;and one of the opposing surfaces of the coupling medium is affixed tothe first surface of the crystal, the unaffixed opposing surface of thecoupling medium being a surface of the sensor means to be urged againstthe body surface.
 3. An apparatus as in claim 2 wherein the unaffixedopposing surface of the coupling medium is contoured to match a contourof the body surface.
 4. An apparatus as in claim 1 wherein:theelectro-optical material is a polymer film having a resilient propertyand further having a reflective coating upon the first opposing surfacethereof; and the conforming means is the resilient property of thepolymer film.
 5. An apparatus as in claim 4 wherein the conforming meansfurther comprises open regions in the polymer film for accommodatingabrupt changes of contour in the body surface.
 6. An apparatus as inclaim 4, wherein the body surface is generally planar with protrusionstherefrom, the polymer film having interior openings for receiving theprotrusions.
 7. An apparatus as in claim 1 wherein the electro-opticalmaterial is a polymer film having a reflective coating upon the firstopposing surface thereof, the first opposing surface thereof beingaffixed to the body surface.
 8. An apparatus as in claim 7, wherein thebody surface is generally planar with protrusions therefrom, the polymerfilm having interior openings for receiving the protrusions.
 9. Anapparatus as in claim 7 wherein the conforming means further comprisesopen regions in the polymer film for accommodating abrupt changes ofcontour in the body surface.
 10. An apparatus as in claim 1 wherein:thecircuit includes conductors corresponding to external circuit nodes, andthe voltage-bearing elements include conductors corresponding tointernal circuit nodes; the electro-optical material is coated with areflective material on the first surface thereof, the first surfacethereof being nearest the body surface; the applying means comprisingmeans for applying a pattern of electrical test signals to the externalcircuit node conductors; and the measuring means comprising:means fordirecting polarized light toward selected regions of the electro-opticalmaterial through a surface thereof opposing the coated surface, theselected regions being electrically proximate the internal circuit nodeconductors; means for receiving light reflecting from the selectedregions; and means for comparing the polarization of the directed lightand the reflected light for each of the selected regions.
 11. Anapparatus as in claim 1 wherein:the circuit includes conductorscorresponding to external circuit nodes, and the voltage-bearingelements include conductors corresponding to internal circuit nodes; theelectro-optical material is coated with a reflective material on thefirst and second opposing surfaces, the first surface being nearest thebody surface and highly reflective, and the second opposing surfacebeing semi-transparent; the applying means comprising means for applyinga pattern of electrical test signals to the external node conductors;and the measuring means comprising:means for directing light towardselected regions of the electro-optical material through thesemi-transparent surface, the selected regions being electricallyproximate the internal node conductors; means for receiving lightemanating from the selected regions, the light being influenced byinterference in the electro-optical material; and means for measuringthe luminous intensity of the emanating light for each of the selectedregions.
 12. An apparatus as in claim 1 wherein:the circuit includesconductors corresponding to external circuit nodes, and thevoltage-bearing elements include conductors corresponding to internalcircuit nodes; the electro-optical material has a plurality of discreteelectrodes disposed on the second opposing surface; the applying meanscomprising means for applying a pattern of electrical test signals tothe external circuit node conductors; and the measuring meanscomprises:means for directing polarized light through one end of theelectro-optical material in a direction generally parallel to the firstand second opposing surfaces thereof; means for biassing selected onesof the discrete electrodes; and means for receiving light from anopposing end of the electro-optical material in a selected receiver, theoutput thereof being indicative of an optical property at a selectedregion of the electro-optical material electrically proximate a nearestone of the internal circuit node conductors.
 13. An apparatus as inclaim 1 wherein the establishing means comprises:means for imposingvoltage levels of the voltage-bearing elements on the first surface ofthe electro-optical material; a transparent electrode disposed on thesecond surface of the electro-optical material opposite the firstsurface; and means for applying a reference potential to the transparentelectrode.
 14. An apparatus as in claim 13, wherein reflective materialis disposed on the first surface of the electro-optical material, andwherein the measuring means comprises:means for directing polarizedlight toward selected regions of the electro-optical material throughthe second surface of the electro-optical material; means for receivinglight reflected from the selected regions; and means for comparing thepolarization of the directed light and the reflected light for each ofthe selected regions.
 15. An apparatus as in claim 1, wherein theelectro-optical material is a polymer having electro-optically activemolecules, and wherein the conforming means comprises means fororienting the electro-optically active molecules of the polymer withinthe sensor means in a direction other than normal to the body surface inthe absence of an electric field.
 16. An apparatus as in claim 15,wherein the polymer includes a first reflective face nearest the bodysurface and an opposing second face, and wherein the measuring meanscomprises:means for directing polarized light toward selected regions ofthe polymer through the second face, in a direction substantially normalto the body surface; means for receiving light reflected from theselected regions; and means for comparing the polarization of thedirected light and the reflected light for each of the selected regions.17. An apparatus as in claim 1, wherein the conforming means comprises aflexible coupling medium having a plurality of conductive channelstherein terminating on first and second opposing surfaces of thecoupling medium, the first surface of the coupling medium being appliedto the first surface of the electro-optical material, and the secondsurface of the electro-optical material being deformable to conform withthe body surface when the coupling medium is urged against the bodysurface.
 18. An apparatus for interfacing a polarimetric test system toa circuit having voltage-bearing elements disposed proximate a surfaceof a body which includes the circuit, comprising:a layer of anelectro-optical polymer conformable to the body surface, the polymerlayer having a first surface for application to the body surface and asecond opposing surface; and a reflective layer disposed proximate thefirst body surface, the second body surface being transparent topolarized light from the polarimetric test system.
 19. An apparatus asin claim 18 further comprising a convergent field lens juxtaposedadjacent the second polymer surface.
 20. An apparatus as in claim 18wherein the reflective layer is integral with the polymer layer anddeformable so that the polymer conforms to the body surface when urgedagainst the body surface.
 21. An apparatus as in claim 18 wherein thereflective layer is affixed to the body surface during manufacture ofthe circuit.
 22. An apparatus as in claim 18 further comprising atransparent conductive layer for connection to a reference potential,the conductive layer being integral with the polymer layer in proximityto the second polymer surface.
 23. An apparatus as in claim 18 whereinthe polymer layer includes electro-optical molecules which are orientedalong a preferred direction relative to the plane of the polymer layer.24. An apparatus as in claim 18, wherein the polymer layer is apatterned film for accommodating contours of the body surface.
 25. Anapparatus for interfacing an interferometric test system to a circuithaving voltage-bearing elements disposed proximate a surface of a bodywhich includes the circuit, comprising:a layer of an electro-opticalpolymer conformable to the body surface, the polymer layer having afirst surface for application to the body surface and a second opposingsurface; and a reflective layer disposed proximate the first bodysurface; and a semi-transparent layer which is semi-transparent toincident polarized light disposed proximate the second body surface. 26.An apparatus as in claim 25 further comprising a convergent field lensjuxtaposed adjacent the second polymer surface.
 27. An apparatus as inclaim 25 wherein the reflective layer is integral with the polymer layerand deformable so that the polymer conforms to the body surface whenurged against the body surface.
 28. An apparatus as in claim 25 whereinthe reflective layer is affixed to the body surface during manufactureof the circuit.
 29. An apparatus as in claim 25 wherein the polymerlayer includes electro-optical molecules which are oriented along apreferred direction relative to the plane of the polymer layer.
 30. Anapparatus as in claim 29, wherein the polymer layer is a patterned filmfor accommodating contours of the body surface.